1. Field of the Invention
The present invention relates to methods and apparatus for recording and playing back ECG signals and more particularly to such methods and apparatus in which the ECG signals are processed to produce a Heart-Rate-Variability (HRV) and late potentials measurements.
2. Description of the Related Art
Prior art systems, known as Holter recorders, record ECG signals which are provided from a patient via electrodes to a small recorder carried by the patient. The ECG signals are continuously recorded on an analog tape typically over a 24 hour period. Several prior art systems use a conventional C60 analog cassette tape of the type typically used to record audio information. In order to do so, the tape must be slowed down approximately 50 times from normal audio speeds. Usually three channels of ECG data are recorded plus a fourth channel which includes a timing signal that is generated by the recorder.
After recording, the tape is played back on a device which runs the tape at high speed and processes the analog signals derived from the tape. Such processing typically includes analog filters which are intended to compensate for dropoff at both the low and high frequencies and for phase distortion which results from the magnetic recording and playback process. The filtered signals are digitized and further processed into a report for analysis by a physician.
One increasingly important analysis of the digitized signals is Heart Rate Variability ("HRV") Analysis. Heart rate variability refers to a variation in the beat-to-beat period over time. Recent cardiological research indicates that decreased variability in normal-to-normal beat intervals can be a prognostic indicator for certain disease states, e.g., congestive heart failure. Theoretically, heart rate variability can be considered an indicator of the responsiveness of the heart to the autonomic nervous system. Heart transplant patients, for example, show dramatically reduced heart rate variability because of the denervation of the heart from the autonomic nervous system.
HRV analysis is based on statistical properties of the beat-to-beat time period measurements generally referred to as the R--R interval sequence. Typically, these statistical properties are generated and analyzed in either the time or frequency domains.
Time domain analysis is based on computation of the mean and one or more measures of variance (such as standard deviation) of the R--R interval sequence. Frequency domain analysis is based on transformations of the R--R interval sequence into a representation of the power spectrum. Typically, three frequency bands are of interest: a low-frequency band (e.g., 0.02-0.09 Hz); a mid frequency band (e.g., 0.09-0.15 Hz); and a mid frequency band (e.g., 0.15-0.40 Hz). Each band is associated with different components of the physiologic control systems affecting heart rate. The low-frequency band is mediated by both sympathetic and parasympathetic nervous systems as well as by mechanisms of thermoregulation and renin-angiotensin system. The mid-frequency band results from baroreceptor reflex regulation of blood pressure. The high-frequency band corresponds to respiratory influence through the parasympathetic system.
Whichever analysis method is employed, the accuracy of the results depends on the accuracy of the underlying R--R interval measurements. In digital playback systems, a fundamental limitation on the accuracy is imposed by the sampling rate of the playback system. See, e.g., Pinna, et al., "Accuracy Boundary in Heart Rate Variability Analysis from Solid State Ambulatory Recordings," Proceedings in Computers in Cardiology, p. 475-478 (1992). The obvious solution is to increase the sampling rate of the digital playback system. Increasing the sampling rate, however, produces a commensurate increase in the cost of the digital playback system electronics. A higher sampling rate results in more data that must be stored and dealt with by the digital system. Therefore, a need remains for a more accurate method of performing HRV analysis without increasing the sampling rate of the playback system.
Another increasingly important analysis of the digitized signals is the detection and analysis of late potentials. Late potentials are low level signals that occur after the end of the conventionally recorded QRS complex. A historical development of late potentials and their clinical use is given in "High-Resolution Electrocardiography: Historical Perspectives," by Benjamin J. Scherlag and Ralph Lazzara, High-Resolution Electrocardiography, pp. 1-15, (1992).
Most late potentials analysis uses a "signal averaging" technique whereby successive ECG signals are summed together and then averaged such that deterministic portions of the ECG signal are reinforced while non-deterministic noise is reduced. The signal averaging technique improves the effective signal-to-noise ratio (SNR) of the resulting averaged ECG signal. The increased SNR lowers the noise floor of the ECG signal such that the low level late potentials can be detected.
The signal averaging technique combines successive ECG signals about a common reference point on each ECG signal. The reference point is used to align the successive ECG signals before summing. Typically, the common reference point corresponds to the R-wave peak. The accuracy of the R-wave detection in a digital system, however, is limited by the sampling rate of the playback system, as described above. The inaccuracy of the R-wave detection causes the ECG signals to be misaligned when they are summed. The misalignment raises the noise floor of the resulting averaged ECG signal making the detection of late potentials more difficult, if not impossible. Therefore, what is needed is an improved method of combining ECG signals which reduces the noise floor to allow accurate detection of late potentials.